Garnets cratonic mantle

Figure 12 Normative garnet Iherzolite mineral abundances (wt.%) versus rock Mg for low-temperature cratonic mantle. Data are open circles, Kaapvaal craton (Boyd and Mertzman, 1987 and references therein) filled diamonds, Tanzanian craton (Lee and Rudnick, 1999) open triangle, Siberian craton (Boyd et al., 1997) cross. Slave craton, northwest Canada (Kopylova and Russell, 2000) filled triangle, northern Canadian craton (Schmidberger and Francis, 1999) and inverted triangle, central Greenland craton (Bernstein et al., 1998).

Figure 18 Normative olivine and opx (garnet Uierzo-Ute norm) versus rock Mg showing low-T cratonic mantle relative to batch melt extraction trends from 3 GPa to 7 GPa (see Figure 6). Open circles are compositions from the Kaapvaal, Siberian, and Slave cratons, and the filled circles are compositions from the Tanzanian, Canadian, and Greenland cratons.

Figure 21 shows site-averaged and grand-average compositions for oceanic, off-craton, and cratonic mantle on plots of normative garnet Iherzolite minerals versus rock Mg. Also shown... 

While all spinel-lherzolite facies suites show remarkably similar compositional trends as a function of depletion, some garnet peridotite xenoliths in kimberlites and lamproites from ancient cratonic lithospheric keels show signih-cantly different trends (e.g., see Boyd, 1989 Chapters 2.05 and 2.08). Most of these xenoliths are extremely depleted extrapolation of the trends back to the PM MgO of 36.7% gives similar concentrations of Si02, EeO AI2O3, and CaO to the spinel Iherzolites (O Neill and Palme, 1998) the difference in their chemistry is due to a different style of melt extraction, and not a difference in original mantle composition. 

Figure 8 Mg/(Mg + Fe) versus Mg/Si for (a) off-craton spinel peridotite xenoliths and (b) cratonic garnet and spinel peridotite xenoliths. Arrows mark the oceanic trend (Boyd, 1989, 1997) defined by abyssal peridotites. Also shown are various estimates for primitive upper mantle (polygons from Table 7).

Figure 20 Primitive mantle normalized extended PGE patterns (including rhenium) for cratonic whole-rock garnet peridotite xenoliths from the Letseng kimberlite (Lesotho) (sources Irvine, 2002 and Pearson et al., 2004).

Figure 38 Nd-Sr isotope variation of clinopyroxenes and garnet in peridotite xenoliths. (a) Compares cratonic and noncratonic peridotite xenoliths with continental crust. Inset shows restricted field for oceanic mantle. Arrow points to a peridotite from Lashaine, Tanzania, that lies at an Sr/ Sr value of 0.83. (b) Compares cratonic peridotites from the Kaapvaal, Wyoming, and Siberian cratons.

Figure 43 CHf versus cnj isotope diagrams for lithospheric mantle peridotite minerals. Kaapvaal peridotite data are ah garnets and clinopyroxenes (Simon et al, 2002). Slave peridotite data are garnets and whole rocks from Schmidberger et al (2002). Salt Lake Crater peridotites (Hawaii), Kilboume Hole and Abyssal peridotites, are from Salters and Zindler (1995). Siberian and Mongohan peridotite Held are chnopyroxene data from cratonic and off-craton peridotites (field taken from Ionov and Weis, 2002). Fields for MORE (N-MORB) and OIB are from Nowell et al (1998). Field for Beni Bousera peridotites from Pearson and Noweh (2003).

Saltzer R. L., Chatterjee N., and Grove T. L. (2001) The spatial distribution of garnets and pyroxenes in mantle peridotites pressure-temperature history of peridotites from the Kaapvaal craton. J. Petrol. 42, 2215-2229. 

Carbno G. B. and Canil D. (2002) Mantle structure beneath the SW Slave craton, Canada constraints from garnet geochemistry in the Drybones bay kimberlite. 7. Petrol. 43, 129-142. 

The effects of post melt-depletion interaction with fluid or melt components in the lithospheric mantle has been extensively documented (e.g. Menzies Hawkesworth 1987, and references therein Harte et al. 1993 Pearson 1999 ) and it is widely accepted that these phenomena dominate the minor element geochemistry of cratonic peridotites. Extensive studies of the effect of metasomatism on the major element chemistry of lithospheric peridotites have also been made (Boyd Mertzman 1987 Keleman et al. 1992, 1998 Walter 1999). To date, most of the discussion has centred around the apparent excess of orthopyroxene, especially in Kaapvaal peridotites. However, major and trace element studies show that it is likely that the abundances of garnet and clinopyroxene are also grossly affected (Burgess Harte 1999 Shimizu 1999). 

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